Move from having each CHERI sandbox class have a single entry point (at
0x8000) to supporting up to four vectors (at 0x200-byte intervals from
0x8000).  Immediately allocate the first two vectors for a run-time
linke entry point (used for constructors and at some point destructors)
and a method-invocation entry point.  Previously, these two entries
points were conflated, and as they have quite different security
properties (in particular, the rtld entry vector trusts the caller), best
to separate them.  There are two unused vectors which are reserved for
future allocation, should we think of a reason (e.g., for debugging or
profiling entry points).

While here, do a moderate amount of tidying up of invocation paths,
including avoiding multiple writes to the per-sandbox memory allocator's
state describing the heap (which added cost to every method invocation),
and testing for constructor/destructor method numbers on each entry
(which similarly added a branch to every method invocation).